There is known a frequency meter incorporated in an automatic excitation voltage regulator and comprising a single frequency measuring channel which includes two resonance circuits. The resonance frequencies of these circuits are 45 Hz and 55 Hz, respectively (cf. Standard Automatic Voltage Regulator of the ARV-SD Type. Technical Description--a publication by the V. I. Lenin All-Union Electrotechnical Institute in Moscow).
This meter measures deviations of the input voltage frequency from a preset value.
It is disadvantageous, however, in that the voltage at its output is dependent not only on the frequency, but also on the magnitude of the input voltage. Another disadvantage is the presence of a filter at the meter's output. The filter has a great time constant, which factor impairs the effectiveness of regulating the excitation voltage of a synchronous electrical machine.
There is further known a frequency meter comprising a power source and a voltage summation unit whose input is connected via a voltage follower to the output of a measuring channel. The latter comprises a zero-crossing detector whose input is connected to the output of a voltage converter, and also includes a storage capacitor and two transistor switches whereof the first has its input connected to the output of the zero-crossing detector and to the input of the second transistor switch which has its output connected to the storage capacitor and voltage follower. The latter has its ouput connected to the input of the voltage summation unit (cf. Automatic Regulation and Control of Power Systems. Transactions of the V. I. Lenin All-Union Electrotechnical Institute, Series 83, Energia Publishers, Moscow, 1977). The meter described above measures deviations of the input voltage frequency from a preset value, while eliminating the effects of the output voltage on the magnitude on the input voltage.
However, the deviation of the input voltage frequency from a preset value is measured only once over a period of this voltage. As a result, at the output of the meter there is produced a signal whose amplitude is indeed proportional to the deviation of the input voltage frequency from a preset value. The problem, however, resides in the fact that this signal is delayed, and the time lag is equal to the input voltage period. There is a smoothing filter at the meter's output, which accounts for an increased time constant of the measuring channel. This factor and the above-mentioned time lag impair the effectiveness of regulating the excitation voltage of a synchronous electrical machine.